1. Field of the Invention
The present invention relates to high voltage fuse construction, and more particularly to support assemblies for supporting fusible elements in high voltage fuses.
2. Description of the Prior Art
High voltage current limiting fuses are well known to the art. Prior art current limiting fuses typically comprise a hollow insulated cylindrical housing which is closed at both ends by metallic end walls. A helically wound fusible element is positioned within the housing and is connected to the end walls. The current limiting fuses are typically filed with an electrically non-conducting insulating material such as silica or quartz sand which surrounds the fusible element. Since it is necessary to assure that the fusible element maintains the proper distance from the sides of the insulated housing and that the individual turns of the fusible element are maintained at a uniform distance to assure that there is no arcing between the turns, prior art current limiting fuses have typically included a support assembly for supporting the fusible element within the fuse housing.
Some prior art current limiting fuses utilize molded or machined ceramic cores for supporting the fusible element. Such ceramic cores are fragile and care must be exercised in handling and storing to assure that the ceramic core is not damaged. In addition, because of the fragile nature of the ceramic cores, the supporting flanges or fingers which position the fusible element must be of substantial size which reduces the amount of sand filler material surrounding the fusible element, particularly where the fusible element engages the supporting fingers. Thus, at those points, there is a reduced ability of the sand to absorb the vaporized fusible element when it fuses thereby restricting fulgurite growth resulting in higher "let through" currents during fuse operation.
Another type of support assembly for a current limiting fuse is disclosed in U.S. Pat. No. 3,863,187 --Mahieu et al. issued Jan. 28, 1975. The support member disclosed in this patent consists of two strips of Mylar plastic which are formed into 90.degree. sectors and welded together using a hot perforating tool. However, fabrication using this technique requires proper vertical and horizontal alignment, and misalignment can result in rejectability of the support member thereby increasing the cost of manufacture. Such support members do not demonstrate good strength characteristics, and since such support members are formed of an organic plastic, a gas is produced due to the decomposition of this material during fuse operation which results in substantial increases in internal pressure in the fuse which may, if excessive, result in rupture of the fuse housing.
Further, difficulty has been experienced in prior art current limiting fuse constructions in terminating the fusible element. Twisting or distortion of the fusible element at the point of connection within the fuse can result in arcing between turns or damage to the fusible element. Further, since the fusible element is typically fabricated from a silver material, electrical connection of the fusible element by welding can result in damage to the fusible element unless the welding temperatures are held to a relatively low level.
In addition, since cost is always a factor for any commercial item, it is desirable to provide a current limiting fuse construction that minimizes the number of dissimilar parts and facilitates rapid low labor cost assembly. Accordingly, it would be a desirable advance in the art to provide a support assembly for a current limiting fuse that reduces the number of dissimilar parts, reduces the labor expense in construction, and maximizes the amount of sand filler material surrounding the fusible element. In addition, it is desirable to assure that the fusible element will be arranged and retained in such a position that flashover from turn to turn does not occur during fuse operation and that the proper concentric alignment of the fusible element with respect to the walls of the current limiting fuse is maintained to prevent localized overheating of the walls.